Sample Material of Our Online Coaching Programme
Subject: General Science
Topic: Gist of Chemistry from NCERT Books
Acid, Base and Salts
1. Acid
- The word ‘acid’ is derived from a Latin word, which means “sour”. The sour taste of most of the fruits and vegetables is due to various types of acids present in them. The digestive fluids of most of the animals and humans also contain acids.
- An acid is a compound, which on dissolving in water yields hydronium ions (H3O+) as the only positive ions. The characteristic property of an acid is due to the presence of these hydronium ions.
- Acids are compounds that contain Hydrogen (Hydrochloric, HCl; Sulphuric, H2SO4; Nitric, HNO3). However, not all compounds that contain Hydrogen are acids (Water, H2O; Methane, CH4). Acids are usually compounds of non metals with Hydrogen and sometimes Oxygen.
- Acids can be classified in various ways, depending on the factors
mentioned below:
1. Classification Based on the Strength of the acid.
2. Classification Based on the Basicity of the Acid.
3. Classification Based on the Concentration of the acid.
4. Classification Based on the presence of Oxygen. - The strength of an acid depends on the concentration of the hydronium ions present in a solution. Greater the number of hydronium ions present, greater is the strength of acid. However, some acids do not dissociate to any appreciable extent in water such as carbonic acid. Therefore, these acids will have a low concentration of hydronium ions.
- Strong Acid: An acid, which dissociates completely or almost completely in water, is classified as a strong acid. It must be noted that in these acids all the hydrogen ions (H+) combine with water molecule and exist as hydronium ions (H3O+). Examples of strong acids are: hydrochloric acid, sulphuric acid, nitric acid etc.
- Weak Acid: An acid that dissociates only partially when dissolved in water, is classified as a weak acid. Most of the molecules remain in solution in molecular form itself in such acid. Examples are: acetic acid, formic acid, carbonic acid etc.
Acids are generally sour in taste. Special type of substances are used to test whether a substance is acidic or basic. These substances are known as indicators. The indicators change their colour when added to a solution containing an acidic or a basic substance. Turmeric, litmus, china rose petals (Gudhal), etc., are some of the naturally occurring indicators.
The most commonly used natural indicator is litmus. It is extracted from lichens . It has a mauve (purple) colour in distilled water. When added to an acidic solution, it turns red and when added to a basic solution, it turns blue. It is available in the form of a solution, or in the form of strips of paper, known as litmus paper. Generally, it is available as red and blue litmus paper.
The solutions which do not change the colour of either red or blue litmus are known as neutral solutions. These substances are neither acidic nor basic.
- Acids are corrosive and can burn flesh and dissolve metal.